Combustors are commonly used in industrial and power generation operations to ignite fuel to produce combustion gases having a high temperature and pressure. Various competing considerations influence the design and operation of combustors. For example, higher combustion gas temperatures generally improve the thermodynamic efficiency of the combustor. However, higher combustion gas temperatures also promote flashback or flame holding conditions in which the combustion flame migrates towards the fuel being supplied by nozzles, possibly causing severe damage to the nozzles in a relatively short amount of time. In addition, higher combustion gas temperatures generally increase the disassociation rate of diatomic nitrogen, increasing the production of nitrogen oxides (NOX). Conversely, lower combustion gas temperatures associated with reduced fuel flow and/or part load operation (turndown) generally reduce the chemical reaction rates of the combustion gases, increasing the production of carbon monoxide and unburned hydrocarbons.
In a particular combustor design, a plurality of tubes may be radially arranged in an end cap to provide fluid communication for a working fluid to flow through the end cap and into a combustion chamber. A fuel may be supplied to a plenum inside the end cap to flow over the outside of the tubes to provide convective cooling to the tubes before flowing into the tubes to mix with the working fluid. The enhanced mixing between the fuel and working fluid in the tubes allows leaner combustion at higher operating temperatures while protecting against flashback or flame holding and controlling undesirable emissions. However, the convective cooling provided by the fuel before entering the tubes may result in uneven heating of the fuel. As a result, temperature and density variations in the fuel flowing through the tubes may produce thermal stress in the tubes and/or uneven fuel-working fluid ratios that adversely affect flame stability, combustor performance, and/or undesirable emissions. Therefore, an improved combustor and method for supplying fuel to the combustor that reduces thermal stress in the tubes and/or temperature and density variations in the fuel flowing through the tubes would be useful.